In the process of electrophotography, a light image of an original to be copied is typically recorded in the form of an electrostatic latent image upon a photosensitive member with subsequent rendering of the latent image visible by the application of electroscopic marking particles commonly referred to in the art as toner. By methods now well known in the art, the residual toner image can be either fixed directly upon the photosensitive member or transferred from the ember to another support, such as a sheet of plain paper, with subsequent affixing of the image thereto.
Problems associated with transferring the latent image to a support, especially the following problem referred to as "toner offset," have been experienced in the field. In these fusing systems, since the toner image is tackified by heat, it frequently happens that a part of the image carried on the supporting substrate will be retained by the heated fuser roller and not penetrate into the substrate surface. This tackified material will stick to the surface of the fusing roller and come in contact with the subsequent sheet of supporting substrate bearing a toner image to be fused. A tackified image which has been partially removed from the first sheet, may transfer to the second sheet in non-image portions of the second sheet. In addition, a portion of the tackified image of the second sheet may also adhere to the heated fuser roller. In this way and with the fusing of subsequent sheets of substrates bearing the toner images, the fuser roller may be thoroughly contaminated. In addition, since the fuser roller continues to rotate when there is no substrate bearing a toner image to be fused there between, toner may be transferred from the fuser roll to the pressure roll. These conditions are referred to in the copying art as "offset." Attempts have been made to control the heat transfer to the toner and thereby control the offset. However, even with the abhesive surfaces provided by the silicone elastomers, this has not been entirely successful.
It has also been proposed to provide toner release agents such as silicone oil, in particular, polydimethyl silicone oil, which is applied on the fuser roll to a thickness of the order of about 1 micron to act as a toner release material. These materials possess a relatively low surface energy and have been found to be materials that are suitable for use in the heated fuser roll environment. In practice, a thin layer of silicone oil is applied to the surface of the heated roll to form an interface between the roll surface and the toner image carried on the support material. Thus, a low surface energy, easily parted layer is presented to the toners that pass through the fuser nip and thereby prevents toner from offsetting to the fuser roll surface. In cases where the toner release surface contains appreciable amounts of silicone to allow sufficient oil wetting, a nonfunctional polydimethylsiloxane oil may be used as the toner release agent. The use of nonfunctional silicone oil with silicone elastomers is known in the art.
According to prior art techniques the toner release agents may be applied to the fuser roll by several delivery mechanisms including wicking, impregnating webs and by way of a donor roll which may comprise a high temperature vulcanized silicone rubber material.
While these silicone elastomer donor rolls have been commercially successful in some commercial applications, they suffer from certain difficulties in that they tend to swell from being in contact with a silicone oil release agent which migrates or is absorbed into the silicone rubber. While a small degree of swelling may be acceptable if it is uniform, failure of such rolls has been observed by excessive swelling over a period of operation wherein the donor roll may actually be twice the original size. Under such circumstances, the silicone rubber donor roll may no longer function in providing a uniform layer of release fluid to the fuser roll.
Further, while donor rolls such as those described in U.S. Pat. No. 4,659,621 have attractive oil delivery capabilities in that they are capable of transporting sufficient quantities of functional release agent to the fuser roll to form the interfacial barrier layer between the fuser roll and the toner, they also tend to swell with the oil penetrating the rubber whereby there may be an interchange of the siloxane oil with the siloxane in the silicone rubber network leading to breakdown of the network and a lower crosslinked network. This reduces the toughness of the silicone rubber barrier layer as more release agent penetrates the surface. This difficulty is particularly pronounced when operating at temperatures in excess of 300.degree. F. Another failure mode is referred to as debonding wherein the swelling of the silicone rubber becomes so significant that it actually delaminates from the core of the donor roll.
Another recent development described in U.S. Pat. No. 5,061,965 to Ferguson et al. describes the use of a donor roll made of a base member, an intermediate comformable silicone elastomer layer, and an elastomer release layer comprising poly(vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene) where the vinylidene fluoride is present in an amount &lt;40 mole %, a metal oxide present in an amount sufficient to interact with polymeric release agent having functional groups to transport a sufficient amount of polymeric release agent to provide an interfacial barrier layer between the fusing surface and the toner. This donor roller suffers from the oil wetting capability between nonfunctional PDMS release agent and the nonreactive donor roller surface, since the invention counts on the polymeric release agent having functional groups to react with the metal oxide which is dispersed in the fluoroelastomer layer.
It would be desirable to have further improvement in the field to overcome the problems of toner offset and donor roll durability.